The present invention belongs to the field of non-destructive testing of structures; more particularly, it relates to a device and a method for the ultrasonic non-destructive testing of structures made of laminated composite material, for example panels used to construct the fuselage or wing members of an aircraft.
Panels of laminated composite material are made by stacking a plurality of layers, formed of fibers, which are held together by a hardened resin.
During the life of an aircraft, a panel of laminated composite material used to form the fuselage or wing members is subject to impacts on its free face, called the front face, due, for example, to bird strikes or tools dropped during maintenance operations. These impacts damage the structure of the panel because they give rise to delamination between its layers. Delamination cannot be detected by simple visual inspection alone.
There is a known ultrasonic testing device, described in European patent application EP1759195, for revealing whether delamination is present in the thickness of a panel after an impact. Using a probe in contact with the front face of the inspected panel, this device emits an ultrasonic pulse and receives the pulse reflected by the rear face of the panel, then compares the form and return time of the reflected signal with the form and return time of a signal called the reference signal, which is reflected by the rear face of the panel and is obtained when the panel is intact. A difference in form and return time between the two reflected signals, if beyond acceptable limits, reveals delamination.
Before this device is used, it has to be calibrated on an intact panel having the same dimensions as the panel to be inspected, in order to obtain the form and return time of the reference signal. Because of this calibration, this device is very effective when the panels to be inspected have a constant thickness, but may give erroneous results if the probe is moved over a length of a panel along which the panel exhibits a decreasing thickness, notably a thickness decreasing in steps.
In fact, the fuselage and/or the wing members of some aircraft are formed by panels of laminated composite materials having a non-constant thickness. At the present time, there are no existing effective devices for the ultrasonic non-destructive testing of these panels. The object of the present invention is to overcome this deficiency, at least in part.